Double Proton Transfer Induced Twisted Intramolecular Charge

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Double Proton Transfer Induced Twisted Intramolecular Charge Transfer Emission in 2-(4'-N,N-Dimethylaminophenyl)imidazo[4,5-b]pyridine Anasuya Mishra, Saugata Sahu, Nihar Dash, Santosh Kumar Behera, and Govindarajan Krishnamoorthy J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/jp404472b • Publication Date (Web): 17 Jul 2013 Downloaded from http://pubs.acs.org on August 1, 2013

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The Journal of Physical Chemistry

Double Proton Transfer Induced Twisted Intramolecular Charge Transfer Emission in 2-(4'-N,N-Dimethylaminophenyl)imidazo[4,5-b]pyridine# Anasuya Mishra, Saugata Sahu, Nihar Dash, Santosh Kumar Behera and G. Krishnamoorthy* Department of Chemistry Indian Institute of Technology Guwahati Guwahati 781039, Assam India

Ph: +91-3612582315 Fax +91-3612582349 Email: [email protected] #

Dedicated to Professor Robert S. H. Liu on his 75th birth anniversary

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Abstract The

spectral

characteristics

b]pyridin-2-yl)benzenamine

of

N,N-dimethyl-4-(4-methyl-4H-imidazo[4,5-

(PyN-Me),

1-methyl-2-(4’-(N,N-

dimetylaminophenyl)imidazo[4,5-b] pyridine (ImNH-Me) and 2-phenylimidazo[4,5b]pyridine (PIP) are investigated to understand the mechanism of protic solvent induced dual fluorescence of 2-(4’-N,N-dimethylaminophenyl)imidazo[4,5-b]pyridine (DMAPIPb). No dual emission is observed from PyN-Me where pyridyl nitrogen is blocked from hydrogen bonding with protic solvents confirms the importance of hydrogen bonding of protic solvents with the pyridyl nitrogen in dual emission of DMAPIP-b. Like DMAPIPb, ImNH-Me also exhibits weak emission and has shorter fluorescence lifetime in methanol. However single emission is observed from ImNH-Me in all solvents including protic solvents. This suggests that the imidazole hydrogen also plays a role in the dual emission process. The longer wavelength emission of DMAPIP-b in water increases with increase in pH of the solution owing to deprotonation of imidazole >NH group. Based on these results the mechanism for the dual emission of DMAPIP-b is proposed.

Keywords: dual fluorescence, ICT, hydrogen bond, proton coupled charge transfer

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1. Introduction Intramolecular charge transfer (ICT) has been a topic of growing interest both in photochemistry and photobiology as it is a possible mechanism for several biological processes and chemical energy conversion. In the donor acceptor system, upon excitation, transfer of charge takes place from the donor to the acceptor moiety which results in the formation of a highly dipolar state namely ICT state and emission takes place from both locally excited state and ICT state. Among various types of ICT process, the twisted ICT (TICT) process is a great area of interest.1,2 In TICT model, the twisted geometry is favored where the donor moiety remains out of plane with respect to the acceptor moiety. Grabowski

et

al.

suggested

that

the

dual

fluorescence

observed

in

dimethylaminobenzonitrile in polar solvents are from the locally excited state and the TICT state.1,2 The factors primarily responsible for the formation and stabilization of the TICT state are polarity, viscosity and hydrogen bonding.2,3 The effects of polarity and viscosity of the medium on the TICT process are well established.2 However the role of hydrogen bonding in the formation and stabilization of the TICT state is still not unambiguously established.3 Studies of hydrogen bonding effects on the TICT process is a very active area of research.3-9 Hydrogen bonding plays a very important role in the proton coupled charge transfer process which often observed in biological assemblies including photo system II and other photoprocesses.10-14 2-(4’-N,N-dimethylaminophenyl)imidazo[4,5-b]pyridine (DMAPIP-b, Chart 1) is a biologically active molecule and it acts as an inhibitor for Aurora-A, Aurora-B and Aurora-C kinases.15 It also has interesting photophysical characteristics. Although, it emits single emission in nonpolar and polar aprotic solvents, it emits dual emission in polar protic medium.16 Therefore, DMAPIP-b was used as a ratiometric probe to study 3 ACS Paragon Plus Environment


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various microheterogeneous environments including protein.17-19 However, despite the fact

that the shorter wavelength emission and the longer wavelength emission are

attributed to normal and TICT emission respectively, the mechanism for the formation of TICT state in protic environment is not clearly understood. Fasani et al. first studied the amino derivative of DMAPIP-b, i.e. 2-(4’aminophenyl)imidazo[4,5-b]pyridine and suggested that the hydrogen bonding of the protic solvent with imidazole nitrogen and its >NH group twist the imidazopyridine ring perpendicular to the rest of the molecule to form the TICT state.20 This contradicts the commonly accepted model that the TICT state is formed by twisting of donor with respect to rest of the molecule and not by the twisting of the acceptor.1,2 Yoon et al. also suggested that the hydrogen bonding of the solvent with acceptor makes it planar with spacer that facilitates the charge flow from the spacer to the acceptor.4,5 In addition in Fasani et al. mechanism20 the pyridyl nitrogen has no role in the formation of TICT state, which contradicts our results that the band maxima and the intensity of the longer wavelength emission are strongly perturbed by the position of the pyridyl nitrogen.16 Therefore, to understand the mechanism of dual emission of DMAPIP-b, we have synthesized and investigated the photophysical characteristics of related molecules (Chart 1) N,N-dimethyl-4-(4-methyl-4H-imidazo[4,5-b]pyridin-2-yl)benzenamine (PyN-Me), N,N-dimethyl-4-(3-methyl-3H-imidazo[4,5-b]pyridin-2-yl)benzenamine or 1-methyl-2(4’-(N,N-dimetylaminophenyl)imidazo[4,5-b]pyridine

(ImNH-Me)

phenylimidazo[4,5-b]pyridine (PIP).


and

2-

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CH3

N

CH3

N

N N

N H

N CH3

N

N

CH3

CH3

DMAPIP-b

PyN-Me CH3

N

N

N N

N

CH3

N

N H

CH3

ImNH-Me

PIP

Chart 1. Structures of DMAPIP-b, PyN-Me, ImNH-Me and PIP.

2. Materials and methods The synthetic procedure of DMAPIP-b is reported elsewhere,16 and those of other molecules are given below: Synthesis of N,N-dimethyl-4-(4-methyl-4H-imidazo[4,5-b]pyridin-2-yl)benzenamine (PyN-Me). PyN-Me was synthesized by irradiating the mixture of DMAPIP-b and methyl iodide by microwave (200 W) using the procedure given in the literature21 and followed by a treatment with ammonia solution.22 The compound was purified by preparative TLC. The identity of the compound was confirmed using NMR and mass spectra. 1H NMR (400 MHz, CDCl3, ppm) δ 7.71 (1H, d), δ 7.53 (2H, d), δ 7.26 (3H, m), δ 6.99 (1H, d), δ 4.19 (3H, s), δ 3.15 (6H, s). From LCMS the molecular mass (M + 1) was found to be m/z: 253.06.



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Synthesis

of

1-methyl-2-(4’-(N,N-dimetylaminophenyl)imidazo[4,5-b]pyridine

(ImNH-Me). DMAPIP-b and methyl iodide (4:9 ratio) was dissolved in a solvent mixture of 3 ml dimethylformamide and 1 ml tetrahydrofuran. Powdered KOH was added and was heated at 40 °C for 24 hour by the procedure reported earlier for alkyl substitution of similar compounds.23 The compound was extracted with dichloromethane and purified by column chromatography. The identity of the compound was confirmed by NMR and mass spectra. 1H NMR (400 MHz, CDCl3, ppm) δ 8.35 (1H, d), δ 8.02 (2H, d), δ 7.77 (1H, t), δ 7.22 (2H, d), δ 6.83 (2H, d), δ 4.00 (3H, s), δ 3.07 (6H, s). From LCMS the molecular mass (M + 1) was found to be m/z: 253.08. Synthesis of 2-phenylimidazo[4,5-b]pyridine (PIP). PIP was synthesized by following the synthetic procedure of DMAPIP-b.16 An equimolar mixture of 2,3-diaminopyridine (Aldrich) and benzoic acid (Aldrich) in polyphosphoric acid was heated at 230 °C for 3 h. The reaction mixture cooled to 100 °C and poured to excess ice cold water and neutralized with base. The compound was extracted with dichloromethane and purified by column chromatography. 1H NMR (400 MHz, CDCl3, and ppm) δ 8.41 (1H, d), δ 8.26 (2H, d), δ 8.15 (1H, d), 7.58 (3H, m), δ 7.30 (1H, m). From LCMS the molecular mass (M +1) was found to be m/z: 196.10. Spectral measurements. Except glycerol (AR grade) all other solvents were HPLC grade from Rankem or Merck, India. The solvents were transparent in the spectral region of interest and were used as received. A Varian Cary 100 UV-visible spectrophotometer and a Horiba Jobin Yvon Spex Fluoromax 4 spectrofluorimeter were used to collect absorption and steady-state fluorescence spectra, respectively. For fluorescence measurements the absorbance of the solutions is kept ~ 0.1 at the absorption band maxima. The fluorescence quantum yields were measured with respect to quinine sulfate 6 ACS Paragon Plus Environment

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in 1 N sulfuric acid (Φf = 0.55).24 Time resolved fluorescence were measured using Edinburgh life spec II instrument, where the fluorescence lifetimes were determined by the method of time correlated single photon counting. A picosecond 375 nm laser diode was used as light source. The data were analyzed by reconvolution method by using the software provided by Edinburgh instrument. Computational calculations. All the molecules are optimized in the ground state using gradient corrected exchange functional of Becke25 and the correlation functional of Lee, Yang and Paar (LYP)26 (B3LYP) using density functional theory (DFT) method and 631G(d,p) as the basis set. Vibrational frequency analyses were performed to confirm the minimum energy nature of the stationary points. Nowadays time dependent density functional theory (TDDFT) method becomes popular for electronic structure calculations in the excited states due to its moderate efficiency and accuracy.27-30 The configuration interaction singles (CIS) predicts the geometries as well as molecular properties quite reasonably and correctly.31-33 TDDFT calculations over CIS optimized geometries are efficient approach in predicting energy parameters for various systems.34-38 We have performed TDDFT/B3LYP/6-31G(d,p) calculations on the optimized ground and excited states geometries to obtain the transition energies. All computational calculations were performed using Gaussian 03 software.39



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3. Results and discussion 3.1. Absorption spectra of methyl derivatives Table 1. Absorption band maxima (λ λmaxab, nm ), fluorescence band maxima (λ λmaxfl, 1 nm) of PyN-Me, and ImNH-Me and DMAPIP-b Solvents λmaxab λmaxfl     Cyclohexane Dioxane DMF DMSO Acetonitrile Methanol Ethanol 1-Propanol 2-Propanol Butanol Glycerol 1

DMAPIP-b

PyN-Me

336, 352

333, 349, 366

323,332

359, 379, 398

373, 393, 419

362, 381, 401

340 346

346, 362 347, 361 347, 362 343, 358 341, 355 343, 356 343, 355 343, 356 343, 357 343, 356

324,333 337 339 331 335 336 336 336 336 343

383 428

401 402 403 395, 430 402 397 398 395 399 406

392 414 421 412 428 421 418 415 414 441

345 350 350 349 346 348 360

ImNH-Me

DMAPIP-b

407 414, 506 413, 494 410, 486 408 407, 475 446

PyN-Me

ImNH-Me

From ref. 16

To understand the role of pyridyl nitrogen and imidazole >NH group in the dual emission of DMAPIP-b, the methyl derivatives, PyN-Me and ImNH-Me (Chart 1) were synthesized. The absorption spectral data of methylated derivatives in different solvents are compared with DMAPIP-b in Table 1. The absorption spectra of all the three molecules are structured in nonpolar solvent. But in polar solvent the vibrational structure in the absorption spectra of ImNH-Me are blurred with a bathochromic shift as that of DMAPIP-b. However, in contrast to other two molecules, the absorption spectra of PyNMe is more structured also in polar solvents. The absorption spectra of PyN-Me is less sensitive towards the polarity of the environment (7 nm shift). Whereas the solvatochromic shift in absorption spectra of ImNH-Me is comparatively larger (20 nm shift) than that of PyN-Me and comparable to that of DMAPIP-b (24 nm shift). Structureless absorption band in polar solvent and larger solvatochromic shift suggest


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charge transfer in ImNH-Me. On the other hand, small solvatochromic shift and structured spectra indicate less charge transfer in PyN-Me. Since, the multiparamateric approach of Kamlet et al. separates the different solvent parameters, it gives information about specific contribution of each solvent parameter.40 Therefore, the absorption and the fluorescence spectral data are analyzed by multiple linear regression analysis to identify the different modes of solvation by employing the following equation. E = E0 + sπ* + aα +bβ Where E is the absorption/fluorescence energy in cm-1, E0 is the value independent of solvent, π* is the polarity/polarizability parameter of the solvents, α is the index of hydrogen bond donating ability of the solvent and β is the index of hydrogen bond accepting capacity of the solvent and s, a and b coefficients are the measure of sensitivities to each individual contributing parameter. The positive sign indicates the destabilization and negative sign indicates the stabilization of the system. The multiple linear regression analysis using the absorption spectral data result in following equations E (DMAPIP-b) = 29860 - 966 π* - 900 α + 84 β (r = 0.98) E (ImNH-Me) = 31150 - 731 π* - 418 α - 802 β (r = 0.96) E (PyN-Me) = 28721 + 474 π* + 540 α - 271 β (r = 0.96) A good linear correlation is obtained for all the three molecules (regression ≥ 0.96). The value of E0 obtained from this approach is also very good agreement with that of the experimental value in nonpolar cyclohexane. As expected due to charge transfer character the polarity/polarizability parameter stabilizes DMAPIP-b and ImNH-Me. In addition the hydrogen bond donating capacity of the solvent also stabilizes both DMAPIP-b and 9 ACS Paragon Plus Environment


ImNH-Me. On the other hand, in PyN-Me where the pyridine nitrogen is blocked and not available for hydrogen bonding, the coefficient of α becomes positive. Therefore, it is clear that the hydrogen bonding of the protic solvent with pyridine nitrogen stabilizes the molecule more than that with the imidazole nitrogen. Large negative values of ‘s’ and ‘a’ for DMAPIP-b indicate that the dipolar interaction and hydrogen bond donating ability of the solvent contribute more for the stabilization of DMAPIP-b than ImNH-Me.

3.2. Fluorescence spectra of methyl derivatives

1

10

0.9 Normalised Intensity

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0.6

0.3

0 360

420

480

540

Wavelength (nm)

Figure 1. Normalised emission spectra of PyN-Me in different solvents: (1) cyclohexane, (2) 2-propanol, (3) 1-propanol, (4) butanol, (5) dioxane, (6) ethanol, (7) DMF, (8) methanol, (9) DMSO and (10) glycerol.


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1

10


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.6

0.3

0 350

450

550

650

Wavelength (nm)

Figure 2. Normalised emission spectra of ImN-Me in different solvents: (1) cyclohexane, (2) dioxane, (3) 2-propanol, (4) DMF, (5) ethylene glycol, (6) n-propanol, (7) ethanol, (8) DMSO, (9) methanol and (10) glycerol.

Like DMAPIP-b, in methyl derivatives also the fluorescence spectra are structured in nonpolar solvents and upon increasing the polarity of the medium it becomes structureless with a red shift (Figure 1 and 2). The spectral shift of ImNH-Me is higher (60 nm) and is comparable to that of shorter wavelength emission of DMAPIP-b (67 nm). From this, it can be inferred that like in DMAPIP-b the locally excited state of ImNH-Me also has charge transfer character. On the other hand, the bathochromic shift is small in the fluorescence spectra of PyN-Me (13 nm) which indicates that PyN-Me is less sensitive towards the polarity of the medium. The fluorescence spectral data are also analyzed by using SCM approach which result in the following equations. E (DMAPIP-b) = 26900 - 2628 π* - 836 α - 593 β (r = 0.93) E (ImNH-Me) = 26453 - 2547 π* - 1922 α + 377 β (r = 0.98) 11 ACS Paragon Plus Environment


E (PyN-Me) = 25210 - 318 π* + 561 α - 476 β (r = 0.67) The fluorescence spectral analysis also shows linear correlation in all the molecules and the values of E0 also match well with the values in nonpolar solvent. As expected the contribution from π* is also high for ImNH-Me and comparable to that of DMAPIP-b.

6600

5200 νss

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3800

2400

1000 0

0.1

0.2

0.3

∆f

Figure 3. Lippert-Mataga plot for ImNH-Me (•) and PyN-Me (■).

Table 2. DFT optimized parameters of DMAPIP-b, ImNH-Me, PyN-Me and PIP Molecules

DMAPIP-b ImNH-Me PyN-Me PIP

Ground state Energy (eV) -20707.0821 -21776.7682 -21776.5134 -17061.3840

Excitation energya (eV)

Emission energya (eV)

µg (D)

ϕ 1b

ϕ 2b

3.82 (3.69) 3.88 (3.84) 3.16 (3.56) 4.23 (4.04)

3.57 (3.27) 3.53 (3.25) 3.09 (3.16) 3.83 (3.65)

5.07 4.77 2.64 1.74

6.77 (3.23) 7.70 (4.32) 0.04 (0.04)

2.54 (0.11) 29.3 (0.53) 0.00 (0.01) 2.28 (0.02)

a

values in parenthesis are experimental value in cyclohexane. bvalues in parenthesis are those of excited state. ϕ1 - the torsional rotation of the dimethylamino group, ϕ2 torsional rotation of the dimethylanilino ring.


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The Lippert-Mataga plot41 was constructed for ImNH-Me, and PyN-Me (Figure 3) by using the relation νSS = [2(µe - µg)/hca3] ∆f + νSS° hereνSS is the Stokes shift, the superscript “o” indicates the absence of solvent, µg and

µe are dipole moments in the ground state and the excited state respectively, a is Onsager cavity radius. The ∆f , the orientation polarizability is defined as ∆f = [(ε - 1) / (2ε + 1)] – [(n2 – 1) / (2n2 + 1)] here ε and n are dielectric constant and refractive index of the solvent, respectively. The excited state dipole moments are estimated from the slope and the ground state dipole moment calculated by DFT method (Table 2). The excited state dipole moments thus obtained for ImNH-Me and PyN-Me are 11.24 D and 5.99 D, respectively. The excited state dipole moment of ImNH-Me is comparable to that of DMAPIP-b (12.20 D, small difference in excited state dipole moment with earlier value16 due the small difference in the ground state dipole moment predicted by ab initio and DFT calculations).

Table 3. Fluorescence quantum yield (Φ Φf) and life time (τf, ns) of methyl derivatives τf Solvent Φf     PyN-Me ImNH-Me PyN-Me ImNH-Me Cyclohexane 0.82 1.22 1.17 Dioxane 0.39 0.89 1.27 1.42 Ethylacetate 0.34 0.29 1.28 1.31 DMF 0.45 0.93 1.41 1.61 DMSO 0.28 0.73 1.05 1.63 Acetonitrile 0.37 0.88 1.42 1.74 Methanol 0.47 0.22 1.78 0.57 Ethanol 0.45 0.81 1.67 1.87 1-Propanol 0.47 0.92 1.62 1.81 2-Propanol 0.43 0.93 1.58 1.87 Butanol 0.48 0.98 1.60 13 ACS Paragon Plus Environment


0.9

0.6 Φf

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.3

0 30

38

46

54

ET(30) kcal/mol

Figure 4. Plot of quantum yield versus ET(30) parameters for ImNH-Me (•) and PyN-Me (♦).

Figure 5. (a) The instrument response function and the fluorescence decays of (b) ImNHMe and (c) PyN-Me in methanol along with fitted curves and residue plot. 14 ACS Paragon Plus Environment

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The fluorescence quantum yields and the lifetime of methylated molecules are complied in Table 3 and the variation of quantum yield with ET(30) parameter is shown in Figure 4. The fluorescence quantum yield of PyN-Me varies very little with the nature of the environment. Whereas, the fluorescence quantum yields of ImNH-Me reduced in protic solvents and lowest in methanol. The fluorescence lifetime also attenuated in methanol (Figure 5 and Table 3). Same behavior is also observed for DMAPIP-b, whose shorter wavelength emission decay time is very low in methanol (0.31 ns) compared to other solvents (1.50 ns in acetonitrile).16 But the fluorescence lifetime of PyN-Me in methanol is also comparable to those in other solvents. From this it can be inferred that the hydrogen bonding of the solvent with pyridyl nitrogen leads to fluorescence quenching. CH3

N N N

N

N CH3

H

H

CH3

N

N

N

H

H

O

O

CH3

CH3

CH3

Scheme 1. Solvent assisted proton transfer. Interestingly both methyl derivatives exhibit single emission not only in aprotic solvents but also in protic solvents. However, DMAPIP-b emits dual emission in protic solvents.16 In PyN-Me, the pyridyl nitrogen is prevented from hydrogen bonding. On the other hand, in ImNH-Me, the imidazole >NH group is substituted by >N-Me and therefore >NH group is not available for hydrogen bonding. The absence of dual emission in PyN-Me and ImNH-Me show that both pyridyl nitrogen and >NH group are 15 ACS Paragon Plus Environment


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vital for the dual emission of DMAPIP-b in protic solvents. One such possibility is transfer of proton from >NH group to pyridyl nitrogen to form tautomer (Scheme 1). Such solvent assisted inter molecular proton transfer in the excited state occurs in several molecules.42-44

3.3. Spectral characteristics of 2-phenylimidazo[4,5-b]pyridine Table 4. Absorption band maxima (λ λmaxab, nm ), fluorescence band maxima (λ λmaxfl, nm) of PIP Solvent λmaxab λmaxfl Cyclohexane 307, 321

324, 340, 358, 374

Dioxane

308, 321

328, 344, 360, 377

Ethylacetate

306, 320

326, 341, 357, 378

Ether

298, 307, 320

324, 340, 357, 374

DMF

308, 321

328, 343, 358, 377

Acetonitrile

307, 319

328, 343, 359, 378

Methanol

307, 320

328, 343, 358, 378

1-Propanol

306 (309) 320

328, 343, 359, 378

2-Propanol

307, 320

327, 341, 359,378

Butanol

307 (309) 321

328, 343, 358, 377

Glycerol

312, 322

332, 348, 362, 382


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1

10


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0.6

0.3

0 315

355

395

435

475

Wavelength (nm)

Figure 6. Normalised emission spectra of PIP in different solvents: (1) cyclohexane, (2) dioxane, (3) ether, (4) ethyl acetate, (5) DMF, (6) methanol, (7) 1-propanol, (8) 2propanol, (9) butanol and (10) glycerol. To verify the proton transfer hypothesis the spectral characteristics of PIP (Chart 1), the molecule without charge donor dimethylamino group, are investigated (Table 4). The absorption and the fluorescence band maxima are blue shifted compared to DMAPIP-b and the methylated derivatives due to the absence of charge donating dimethylamino group. The solvatochromic shift is also very small compared to other molecules and the fluorescence spectra also posses well resolved vibrational structure (Figure 6). Markedly no dual emission is observed in any solvent including protic solvents such as methanol. Despite the fact both pyridyl nitrogen and >NH group are freely available for hydrogen bonding, PIP exhibits single emission. Therefore, it can be concluded that the electron donating group is absolutely necessary for the dual emission in protic solvents and the longer wavelength emission originates from the charge transfer state. Further, strong positive solvatochromic of the longer wavelength emission of 17 ACS Paragon Plus Environment


DMAPIP-b and the higher dipole moment of it’s emitting state (section 3.5) also substantiate the conclusion that the state is a charge transfer state.

3.4. Effect of deprotonation

13.57

pH

Absorbance

0.18

9.20 9.50 10.17 10.91 11.20 11.64 12.14 12.57 12.94 13.17 13.41 13.57

9.2

0.12

0.06

0 280

330

380

430

Wavelength (nm)

Figure 7. Absorption spectra of DMAPIP-b for neutral-anion equilibrium.

13.57

8.93


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0.6

13.57

8.93

0.3

0 360

460

560

660

Wavelength (nm)

Figure 8. Normalised emission spectra (λexc = 349 nm) of DMAPIP-b for neutralmonoanion equilibrium.


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To get further insight about the role of >NH proton in the dual emission, the neutral-anion equilibrium of DMAPIP-b was investigated. At pH 9.0 the molecule is in neutral form.16 Upon increasing the pH the absorption spectrum is blue shifted with hyperchromic effect (Figure 7). DMAPIP-b has only one acidic proton (>NH). The spectral changes are consistent with the deprotonation of >NH group to form an anion. The blue shift is also observed in absorption spectra of similar molecules when the imidazole >NH group is deprotonated.45,46 The effect of pH on the fluorescence spectra is shown in Figure 8. The fluorescence spectrum of DMAPIP-b has emission band in water at 451 nm with long tail. With increasing pH of the solution, the normal emission band is blue shifted. But the most interesting change is the emergence of longer wavelength emission upon deprotonation. At higher pH, two different lifetimes are observed for DMAPIP-b when monitored at 425 nm and 545 nm (Table 5). Compared to shorter wavelength when monitored at longer wavelength, the relative amplitude of the short lifetime species decreases while that of the long lifetime species increases. This indicates that the longer wavelength emitting state has longer lifetime and the shorter wavelength emitting species has shorter lifetime. This is consistent with the lifetimes of the neutral form of DMAPIP-b.16 The observation of clear dual emission in anion suggests that the deprotonation may be one of the important step in the formation of longer wavelength emitting state in DMAPIP-b. Table 5. Fluorescence lifetime (τ, ns) of the monoanionic form of DMAPIP-b and the values in parenthesis are relative amplitude Monitoring τ1 τ2 χ2 wavelength (nm) 425 0.37 (30.44) 2.79 (69.56) 1.05 545

0.39 (20.14)

2.81 (79.86)


0.95


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

3.5. Double Proton transfer induced TICT emission The studies suggest that both pyridyl nitrogen and imidazole >NH group play important role in the dual emission of DMAPIP-b in protic solvents. The charge donor is absolutely essential and the longer wavelength emission is from charge transfer state. The dipole moment of the longer wavelength fluorescence emitting state was estimated by both solvatochromic (24.6 D) and thermochromic (25.5 D) methods.47 The value is much higher than that of shorter wavelength emitting state (12.2 D). Based on the theoretical calculations, it was reported in the literature that the dipole moment of the TICT state is much higher than the planar ICT state.48 Much higher dipole moment of the longer wavelength emitting state of DMAPIP-b clearly suggests that the ICT state is a TICT state. The enhanced dual emission in the anion reveals that the formation of TICT state in protic solvent involves the deprotonation of imidazole >NH proton. Based on these results we hypothesize that the formation of the TICT state is induced by solvent assisted (double) proton transfer from imidazole >NH to pyridyl nitrogen. The dissociation of proton from >NH group will produce a negative charge on the pyridyl nitrogen by resonance (Scheme 2), abstraction of proton by pyridyl nitrogen results in proton transfer. It is reported that the excited state intramolecular proton transfer is accompanied by a torsional rotation of the tautomer leads to formation of twisted charge transfer state in number of molecules.38,49-52 However, in contrast to DMAPIP-b, the formation of tautomer in those systems is an intramolecular process and torsional motion of the tautomer often directs to nonemissive charge transfer state.


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CH3

N

CH3

N

N

N N

N

CH3

N

N

CH3

Scheme 2. Resonance stabilization of the anion form of DMAPIP-b.

Relative energy (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


X

3.5363

3.3670

S1

3.6282

3.3529

S0

0.8355

0.6919 0.2445 0

Twisted (ϕ ϕ1 = 900)

Planar (ϕ ϕ1 = 00)

Planar

CH3

N

CH3

N

N N

N

H

H

Twisted (ϕ ϕ2 = 900) CH3

N N

N CH3

ϕ1

N

N

CH3

H

H

N

N

ϕ2

CH3

H

H

O

O

O

CH3

CH3

CH3

Scheme 3. Energy level diagram of different tautomer of DMAPIP-b. To verify the hypothesis the structures of methanol-DMAPIP-b complexes are optimized and the energy level diagram was constructed (Scheme 3). The energy level diagram also shows that the excited state energy of the planar tautomer is less than that of normal form. The TICT state can be formed by rotation of either dimethylamino group or



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dimethylaminophenyl ring. The calculations indicate that the energy of the dimethylaminophenyl ring twisted state is higher than that of the planar state and the energy of the twisted state is even higher than that of the locally excited state. Therefore, the formation of the TICT state by twisting of the dimethylaminophenyl ring is not a favored process. On the other hand, the energy of the dimethylamino group twisted state of the tautomer is lower than that of the planar state. This suggests that the TICT state is formed by the twisting of the dimethylamino group rather than the dimethylaminophenyl ring. The experimental emission energy of the longer wavelength fluorescence (2.46 eV)16 is in better agreement with the emission energy predicted from the twisted state (2.66 eV) than the predicted energy from the planar state (3.12 eV).16 This supports the proposed mechanism. Yoon et al. reported that the excited state proton transfer promotes the formation of TICT state in 4-N,N-dimethylaminosalylic acid and 4-aminosalylic acid. 53,54

They suggested that 4-aminosalylic acid which has weak electron donating group

also emits TICT emission due to formation of phototautomer. They also demonstrated that the TICT emission of 4-N,N-dimethylaminosalylicacid is enhanced inside cyclodextrin cavity due to enhancement in the intramolecular hydrogen bond which favors the formation of phototautomer. Recently Guchhait et al. also proposed such proton transfer assisted charge transfer in some photochromic Schiff bases.55 Nonetheless, in DMAPIP-b the formation of tautomer is a double proton transfer process assisted by protic solvent molecule not an intramolecular process.

4. Conclusion Both PyN-Me and ImNH-Me exhibit single emission not only in aprotic solvents, but also in protic solvents. The absence of dual emission in PyN-Me where pyridyl nitrogen 22 ACS Paragon Plus Environment

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is prevented from hydrogen bonding with protic solvents emphasizes that the hydrogen bonding with pyridine nitrogen is crucial for the dual emission of DMAPIP-b in protic solvents. On the other hand, the absence of dual emission in ImNH-Me suggests that the hydrogen bonding with hydrogen of imidazole >NH group is also playing an important role in the process. The absence of dual emission in PIP substantiates the conclusion that the longer wavelength emitting state is a TICT state. The enhancement of longer wavelength emission at higher pH illustrates that the formation of charge transfer state involves the deprotonation of imidazole >NH hydrogen. The dual emission of DMAPIPb in protic solvents is due to the formation of TICT state that is induced by double proton transfer. The TICT state is formed by the twisting of the dimethylamino group. The energy level diagram obtained by theoretical calculation also supports the hypothesis.

Acknowledgement The authors acknowledge department of science and technology (DST), India and council of scientific and industrial research (CSIR), India for the financial support. The authors also thank the Central Instruments Facility (CIF), Indian Institute of Technology Guwahati for the Life Spec II and NMR instruments.

Supporting Information: Full authors list for references 28 and 39. This material is available free of charge via the Internet at http://pubs.acs.org.

References



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Table of Content

0

S0

ϕ1 ∼ 90

0

ϕ1 ∼ 0 Planar CH3

N

N

N

H

H

CH3

N N

N CH3

N

N

H

H

O

O

CH3

CH3

ϕ1

CH3

TICT emission


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Double Proton Transfer Induced Twisted Intramolecular Charge

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